Unvailing the King of Fat Bike Design Puzzle

The rear offset design of the Pugs and Moonlander...I have never been able to get my head around the reason outside of the ability to swap wheels front and rear, and it is what was handy at the time.

If one desires to run a 29er wheelset on a fat bike, it always stuck in my mind that handling would suffer over a purpose built 29er with the same geo, due to the differences in dropout width.

To make my point with an extreme example...lets increase the rear OLD to 10 feet. The bike would then be neigh impossible to steer...correct? It would take more input into the lean to get the same otherwise response?

Also with a 135mm OLD up front, all the adjustments in the world to the head angle or offset would be in vain. The only saving grace would be to add weight over the front axle...a lot?

What I am driving at is...is there something to be lost with the coming advent of 186/190 OLDs? If the above logic holds water something has to give.

That being the case; how do you design around this...enter the Pugs and Moonlander?

If the above logic is true is it also true that an offset rear dropout...to the right...also decreases the lean effort to the left over the right? But then comes this puzzle...it is naturally easier to lean left over right, so how are we to know just what is in play? But it also seems logical that there has to be an over riding truth regardless of what our brain says.

What if in an offset design the drive side was moved to the left...? The larger question is how noticeable would any of this be?

If all of the above is true does this not increase the value of an offset rear in our goal to run wider rubber?

Why would dropout width make handling worse? If anything it would make handling better, due to stiffness.

Why would dropout width affect head angle and trail? the width of the dropouts affects neither the trail or head angle, where as, the trail and head angle do affect each other when you change one of them.

Are you sure you're not high? I mean, why is it naturally easier to lean left than right? That's nonsense, everyone is different.

I think that either, I am confused, or you are confused. Could you perhaps clarify your thoughts to alleviate my confusion? I don't see any reason that wide hubs or offset design would affect the handling. The centreline of the wheel is still in line with the centreline of the bike.

Bikes of yore tried drive trains on the left and it works jsut fine, but the right side drive train became standard. I don't think that any of that makes any real difference to riding. I am more comfortable leaning left than right, but that is individual. Some will be more comfortable to the right. But even still the difference for me is negligible.

Hub width does not affect handling except where it might increase weight or stiffness. The width itself is not part of the handling or fit geometry.
Offset vs symmetric also does not affect handling beyond what effect it might have on wheel stiffness.
A fatty 29er may suffer some compared to a designed 29er but not because of the hub width. It is because of compromises needed to clear the fat tires. Particularly using 100mm bottom bracket and extra wide chain stays. But these are mostly ergonomic issues not handling issues.
Building a 29er wheel on an offset hub does involve some compromises because the wheel must be dished to accommodate the offset. The Moonlander 28mm offset is probably beyond what you can do with 29er rim.

The ability to swap front and rear wheels is not the main reason for the offset design. Rather, it's to allow the chain to clear the tire with a standard 135mm rear hub. Because the hub is not centered in relation to the rim, the asymmetrical frame becomes necessary to center the rim in relation to the main tubes of the frame (and the rider). An asymmetrical fork and offset front wheel are not required at all.

To the OP: From what I gather of your overall understanding of bicycle dynamics, you should just revert to listening to the advice of those with experience, and when it comes to concepts of offsets, and asymmetrical designs, just trust us all that all of the designs out there work great for what the manufacturers are suggesting they be used for.

as far as finding the right bike for what you want to do, I would suggest just stating what you want to do with the next bike you might get in a thread, and try the bikes suggested on a trail if possible, and not worry at all about what numbers are involved in the geometry, or what the bike looks like.

On the idea of wider hubs affecting handling, iwould say that a 29er wheelset built with 170mm, and 135mm hubs would handle better due to stronger stiffer wheels, and that for heavier stronger riders, I would suggest them even on a 29er without fat tires. I have destroyed a 100mm hub 29er wheel from riding too fast through a turn without even hitting anything but smooth dirt.
I think 135mm front forks should be specified on 29ers for clydesdale type riders.

and as far as 17.5mm offset 135mm 29er wheels, a rear wheel probably has less dish(more even spoke tension) than a standard 29er geared rear wheel, as the rim is dished to the opposite side from normal.

Not sure if my failure to follow some is due to them not following...me. Perhaps due to the limits of the internet...that sounds safe!

Let me try it from a different angle. Longer trail...does what? A longer stem...does what?
A longer bar does...what? Why would not a wider (longer) axle have the same effect?

I get that regardless of the design, the wheels are by necessity centerlined. The wheel, the contact patch is like a fulcrum point for the axle. The Axle is like a plank on a teeter totter, or a lever if you will. In effect, if you try to do a one man balance act on a teeter totter, say your feet are fixed equal distant from the fulcrum point.

The wider the symmetrical dropout, the farther apart your feet. An offset design affords a closer stance, at the expense of being asymmetrical to the fulcrum point. This poses an adjustment in the balance act, but I see it as being neutral overall in affect, just different.

If 6" tires are produced, what are we looking at...206 hubs and 150's with 28mm offset? It is just that it boggles my mind to think that hub width can change in a vacuum with regards to feel of handling.

My aim in this thread is to explore, what ever the facts are is what they are, understanding the forces at work better is the goal. Which brought about the question; With hub widths at an extreme, would it be a mistake to believe that a 29er set would prove just as satisfactory as in a frameset built for 4" tires?

As for the left turning thing...oval track racers go in what direction? Why? If by default of choice, I submit there is a reason behind that choice by default though unstated and unrecognized.

And no offence meant...I don't trust anyone who says..."Trust Me" I am not concerned with adhering to convention if I see it as to my advantage to buck it..."Trust Me" LOL

Re: Unvailing the King of Fat Bike Design Puzzle

A wider hub does not necessitate any geometry or ergo changes.
If tires get much wider you will need wider bottom bracket which will affect ergos.
135 Offset frames and 170 frames use the same BB size and usually same chain stay length.

Re: Unvailing the King of Fat Bike Design Puzzle

If it helps think of bicycle geometry as 2d. That is the only dimensions that affect handling.
The axle is just a point on the 2 D plane. Width does not matter.
It may affect wheel and frame stiffness.

Skinny bikes don't have so much variation in tyre size, tyre pressures and rim size. On a fatbike, change just one of those factors and you have a different handling bike.

I'm not keen on the current crop of 135mm forks. We're still using axles with a diameter designed for the size of tyres they had in the 1890s, and yet a fatbike has tyres the size of a motorbike with the resultant feedback into the frame. I would like to see large diameter axles on fat forks and until then I'll stick to as narrow as possible, ie 100mm.

The most precise handling I've found is when I had a very steep head angle which is contrary to the more accepted view, but that was only tested with a narrow range of tyres and rims.

I suspect the most important factor may not be trail or head angle but the combination of flop and the tyre profile (ie how rim size affects tyre shape). I'll probably get round to testing that some time this year.

As little bike as possible, as silent as possible.
Latitude: 57º36' Highlands, Scotland

Skinny bikes don't have so much variation in tyre size, tyre pressures and rim size. On a fatbike, change just one of those factors and you have a different handling bike.

Bar width and stem length can be tweaked to adjust steering feel...are not contact patch width and axle length much the same? A bike is also steered through the seat no?

A concern that I have is that as axle width at the rear is increased while the front remains constant is that there could be a pushing effect generated on the front which could be adverse.

I do not foresee any proof for my concerns forthcoming unless an engineer can bring clarity. But until then...forewarned is fore armed? If any possible downside is outweighed by whatever upside for a given individual...then go for it...you now have the tools to make a better choice?

Take Beth...with her experience with her mishaps...what I took away for myself is that at my age, it might be best to stick to fat tires alone. Going from fat to skinny, opens myself up to overlooking possible hazards due to a change in tire width. It could end my riding days at worse and in any event it now takes longer to recover, the risks outweigh the benefits.

So that's where I am at with all of this...thanks to all for being there to prod me on.

Bar width and stem length can be tweaked to adjust steering feel...are not contact patch width and axle length much the same? A bike is also steered through the seat no?...

Yes, you can change the feel and a good setup can mask imperfect geometry up to a point. The human body is incredibly adaptable and with a bit of practise you can ride what seemed unrideable. Which is why we can have different fashions in bike configuration and geometry from decade to decade instead of an absolute fixed design. For an example find an old upright bike with rod brakes and try riding it on singletrack. Feels all wrong, but that's what our ancestors were on 100 years ago and riding them around the world on dirt roads.

As little bike as possible, as silent as possible.
Latitude: 57º36' Highlands, Scotland

There is merit in the OP, if not ultimate clarity. It does do good to reflect on the idea offered, if not the surmise, and to look further for answers.

Originally Posted by Velobike

There are a lot of variables affecting fatbike handling.

Skinny bikes don't have so much variation in tyre size, tyre pressures and rim size. On a fatbike, change just one of those factors and you have a different handling bike. I suspect the most important factor may not be trail or head angle but the combination of flop and the tyre profile (ie how rim size affects tyre shape). I'll probably get round to testing that some time this year.

I am thinking that I agree. I noted a distinct self steer on my first Fatty. This was something I have not really encountered before, at least to this degree. I was trying to figure out what exactly was causing it, especially since I was the one building the frames, and as such, always seeking to improve the "product." My own thinking and experience since, added to those of others on this forum, has lead me to believe that the tire is the single largest factor in handling.

I have very recently come clear to the notion that some of what I was feeling was simple gyroscopic effect, and it has become quite noticeable to me now that I have separated it from other phenomena. The simple fact that you have all that added mass out there spinning about its axis explains that neatly.

So too do I believe that the width of the tire is a larger lever acting upon the steering, in a radial sense. The extra width of the tire gives surface grip far outside the width of a normal MTB tire, and my thinking is that this is also responsible for some of the pull felt at the bar. Add to that the notion that the tire certainly is deforming away from the centerline of the wheel under these loads (see attached), and it seems that the effect would be further magnified.

Tire pressure of course has something to do with that, but in my experience I'm thinking that takes a back seat to tread design. It has been noted by many, including myself, that the HuDu is very prone to self steer, where you do not necessarily hear that about the Nate; a tire with a decidedly more aggressive tread. This mirrors my experience.

I'm not keen on the current crop of 135mm forks. We're still using axles with a diameter designed for the size of tyres they had in the 1890s, and yet a fatbike has tyres the size of a motorbike with the resultant feedback into the frame. I would like to see large diameter axles on fat forks and until then I'll stick to as narrow as possible, ie 100mm.

Are you taking exception based on perceived structural weakness, or do you feel that you can actually sense too much flex from the wider hubs? I also am not overly enthused by not having the option of some sort of through axle for the front, and if I were smart enough to figure out how to get one of these new 12mm T/A rear hubs on the front, I'd do it. No, I've never broken a skewer, but at the same time, it sure SEEMS impossibly frail.

Attached Thumbnails

Most people ply the Well Trodden Path. A few seek a different way, and leave a Trail behind.
- John Hajny, a.k.a. TrailMaker

I get that regardless of the design, the wheels are by necessity centerlined. The wheel, the contact patch is like a fulcrum point for the axle. The Axle is like a plank on a teeter totter, or a lever if you will. In effect, if you try to do a one man balance act on a teeter totter, say your feet are fixed equal distant from the fulcrum point.

The wider the symmetrical dropout, the farther apart your feet. An offset design affords a closer stance, at the expense of being asymmetrical to the fulcrum point. This poses an adjustment in the balance act, but I see it as being neutral overall in affect, just different.

If 6" tires are produced, what are we looking at...206 hubs and 150's with 28mm offset? It is just that it boggles my mind to think that hub width can change in a vacuum with regards to feel of handling.

Please feel free to jump in with more comments.

I think one of your mistakes is to think that the fulcrum of your teeter totter moves over as the hub is offset to the side. The balance point or fulcrum is the tire contact to the ground. Your connection is your feet on the pedals. The connection between those two points does not matter as long as it is rigid. Obviously if the connecting structure is made super wide so it hits the ground as you lean that would be a problem and if it was offset way to one side so there was lots more mass on one side that could be a problem. At the sizes we are talking about between a 29er wheel and a fat wheel it does not matter.

You can't throw axle length in with the stem or bar length changes. By changing bar or stem length you are changing the distance from the axis of rotation which affects leverage and handling. No matter how wide the axle is, as long as the wheel is still centered there is no change about the axis of rotation.

...Are you taking exception based on perceived structural weakness, or do you feel that you can actually sense too much flex from the wider hubs?...

I've tried 3 different 135mm forks. On each of them heavy braking produced an element of instability which was not obvious on the 100mm fork. The best description I can give of it was that I was feeling squirm.

I haven't done any measurement comparison of the actual (if any) flex for the various forks/wheel combinations, so this is just an opinion. If someone was to do this it would be an interesting comparison.

The likes of a truss fork may compensate for inadequacies in the axle.

In the meantime I'll stick to narrow OLDs if possible.

Originally Posted by tfinator

This seems like a troll effort. Fun reading,though!...

No, it's a point worth raising. Sand rat likes to philosophise.

As little bike as possible, as silent as possible.
Latitude: 57º36' Highlands, Scotland